Artificial planar lipid bilayers serve as simplified models of biological membranes and are widely used for the electrical characterisation of ion-channels and protein pores. Ion-channels are a diverse group of membrane proteins that selectively control the movement of specific ions across cell membranes, establishing voltage and electrochemical gradients that are fundamental to a wide variety of biological processes. In humans, ion-channels regulate everything from heartbeat and muscle contraction to hormone secretion and the release of neurotransmitters. Defective ion channel function is implicated in a growing list of disorders, including cardiac arrhythmia, periodic paralysis, epilepsy and diabetes (Ashcroft, F. M. 2000, Academic Press, San Diego; Ashcroft, F. M. 2006, Nature 440, 440-447; Kass, R. S. 2005, Journal of Clinical Investigation 115, 1986-1989). Protein pores are non-specific channels that allow molecules to pass across cell membranes. Protein pores can be exploited for many applications such as molecular sensing (Bayley, H. et al., 2000, Advanced Materials 12, 139-142; Bayley, H. & Cremer, P. S. 2001. Nature 413, 226-230) and DNA sequencing (Kasianowicz, J. J. et al., 1996. Proceedings of the National Academy of Sciences of the United States of America 93, 13770-13773; Howorka, S. et al., 2001. Nature Biotechnology 19, 636-639; Astier, Y. 2006. Journal of the American Chemical Society 128, 1705-1710).
Single-channel recording (SCR) of individual proteins is a powerful means of studying channel protein function (Sakmann, B. & Neher, E. 1995. Plenum Press, New York; London). Single-channel recording measures changes in ion-current through single protein channels, and can examine voltage dependence, gating behaviour, ligand binding affinity, and ion selectivity at the single-molecule level. Consequently, single-channel recording can help determine the molecular basis of an ion-channel disease. It is also an important technique for the development of new drugs specifically targeting channelopathies, and for screening other medicines for unwanted side-effects (Ashcroft, F. M. 2006. Nature 440, 440-447; Roden, D. M. 2004. New England Journal of Medicine 350, 1013-1022). Advances in these areas require much higher throughput assays of ion-channel behaviour than are currently available.
Single-channel recording typically uses either patch-clamping (Sakmann, B. & Neher, E. 1984. Annual Review of Physiology 46, 455-472) or artificial planar lipid bilayers (Mueller, P. et al., 1962. Nature 194, 979-980; White, S. H. 1986. ed. Miller, C. Plenum Press: New York). Although other methods may also be used, including excised-patch, tip-dip and on-chip methods.
Patch-clamping of whole cells is a versatile and sensitive means of examining channels, but is time-consuming and often complicated by the heterogeneous nature of cell membranes. In contrast, artificial planar lipid bilayers control the constituents of the system and can be used to study purified proteins. Planar lipid bilayers are usually formed either by painting, where a solution of lipid in an organic solvent is directly applied to an aperture separating two aqueous compartments (Mueller, P. et al., 1962. Nature 194, 979-980; White, S. H. 1986. ed. Miller, C. Plenum Press: New York), or variants of the Langmuir-Blodgett technique, where two air/water monolayers are raised past an aperture (Montal, M. & Mueller, P. 1972. Proceedings of the National Academy of Sciences of the United States of America 69, 3561-3566). Although widely used, planar lipid bilayers are difficult to prepare, and their short lifetime prohibits their use in many situations.
Alternative emulsion-based approaches to forming bilayers have also been proposed (Tsofina, L. M. et al., 1966. Nature 212, 681-683), where bilayers are created between aqueous surfaces immersed in a solution of lipid in oil. When immersed in an immiscible lipid/oil solution, aqueous surfaces spontaneously self-assemble a lipid monolayer (Cevc, G. 1993. Phospholipids handbook, ed. Cevc, G., Marcel Dekker, New York); Seddon, J. M. & Templer, R. H. 1995. eds. Lipowsky, R. & Sackmann, E., Elsevier, Amsterdam, Oxford), and when monolayers from two aqueous components are brought into contact they can ‘zip’ together to form a lipid bilayer (Tien, H. T. 1974. M. Dekker, New York; Fujiwara, H. et al., 2003. Journal of Chemical Physics 119, 6768-6775). Recent studies have shown that microfluidic flows (Malmstadt, N. et al., 2006. Nano Letters 6, 1961-1965; Funakoshi, K. et al., 2006. Analytical Chemistry 78, 8169-8174) and droplets (Funakoshi, K. et al., 2006. Analytical Chemistry 78, 8169-8174; Holden, M. A. et al., 2007. Journal of the American Chemical Society p 8650-5) can be contacted in a lipid/oil solution to create bilayers suitable for single-channel recording experiments.